Abstract: The reasons of cracks in the tee of cyclopentane feed pipe and cracking occurred after repair welding were analyzed by means of macrographic examination, thickness testing, hardness testing, chemical composition analysis, metallographic examination and so on.The results showed that the cracks appeared in the tee during processing and manufacturing, and expanded and leaked under the effect of installation stress and piping stress.Cracking occurred again in crack area caused by welding stress after repair welding.Finally, the suggestions were put forward for welding repair and testing of the same type tee.